Products made from base webs such as bath tissues, facial tissues, paper towels, industrial wipers, food service wipers, napkins, medical pads, and other similar products are designed to include several important properties. For example, the products should have a soft feel and, for most applications, should be highly absorbent. The products should also have good stretch characteristics and should resist tearing. Further, the products should also have good strength characteristics, should be abrasion resistant, and should not deteriorate in the environment in which they are used.
In the past, many attempts have been made to enhance and increase certain physical properties of such products. Unfortunately, when steps are taken to increase one property of these products, other characteristics of the products may be adversely affected. For instance, the softness of non-woven products, such as various paper products, can be increased by several different methods such as by selecting a particular fiber type or by reducing cellulosic fiber bonding within the product. Increasing softness according to one of the above methods, however, may adversely affect the strength of the product. Conversely, steps normally taken to increase the strength of a fibrous web typically have an adverse impact upon the softness, the stiffness, or the absorbency of the web.
In order to produce products of desired characteristics, and to ensure the processes that create these products runs smoothly, properties of the web during production are often monitored. One such property that allows for the characteristics of the web to be controlled is the tension of the web. Properties which can be controlled based on the tension of the moving web include but are not limited to strength related properties such as machine direction modulus, basis weight, moisture, and properties that relate to softness. However, other measurements of different properties of the web must sometimes be made in order to control some of the aforementioned properties.
Measurement of the tension is also helpful in preventing breaks of the web during production of a paper product. A papermaking machine can be modified if the recorded tension is high enough to subject the web to breaking. Such a modification of the production process to avoid these web breaks can prevent downtime of the papermaking machine.
Various ways of measuring the tension of a moving web are known in the art. For instance, U.S. Pat. No. 4,833,928 discloses a non-contacting tension measurement method by which a microphone induces sound waves in the web which are subsequently detected by microphones installed close to the web. Additionally, the tension in a moving web may be measured by contacting the web with rolls that have force transducers or load cells mounted therein to measure the tension in the moving web. Such a way of measuring the tension of a moving web is known in the art as a contacting tension measurement system. A non-contacting method to measure the tension in a moving web exists by forming a wave on the web by means of a blast of compressed air. The subsequent wave is then measured and this measurement is used to calculate the tension of the web.
Another patent indicative of a process and apparatus that measures tension in a moving web is U.S. Pat. No. 3,854,329. This patent is directed towards a non-contacting method that makes use of a loud speaker and a microphone to create and measure a vibration in the moving web to determine the tension.
A method of measuring the tension in a moving web when the tension is low, and the web speed is high is unique to this application. Additionally, a process of measuring the tension on a wide commercial tissue machine at both high speed and low tension is unique to the present application. Also, a method that is suitable for commercial production is further unique. Current machines do not disclose a way of measuring the tension on a moving web when the instability index of the web is greater than 0.5. Current machines measure the tension by using either a contacting method that typically will not work on a tissue web at high speeds, or by means of a sonic method that is only suitable for stiff webs where the instability index is low, typically less than 0.5.
Objects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.
The present invention provides for a process for determining the tension in a moving web. The process includes the steps of providing a web that is moving at a determined speed. The web has a determined basis weight. A wave is created in the moving web, and the speed of propagation of the wave is determined. The tension on the moving web is determined through a mathematical relationship between the wave speed, the basis weight of the web, and the speed of the web. The instability index of the web is greater than or equal to 0.5.
The process also includes an exemplary embodiment where the instability index of the web is greater than about 0.8.
The present invention also encompasses a process for producing a paper web that has substantially uniform properties. The process includes the steps of providing a moving paper web, and the determination of the tension on the paper web while the web is moving. The process also includes the step of adjusting a process condition of the web that effects modulus based on the determined tension to produce a web that has more uniform properties. The instability index of the web is greater than or equal to 0.5.
The present invention also includes an exemplary embodiment as immediately discussed where the step of determining the tension on the paper web includes the steps of creating a wave in the moving paper web and the determination of the speed of the propagation of the wave. Also, the step of determining the tension includes the step of calculating the tension on the moving web through a mathematical relationship between the wave speed, the basis weight of the paper, and the speed of the web.
The present invention also includes an embodiment as previously discussed where the step of determining the tension on the moving web occurs when the instability index of the web is about 0.8 or higher.
Another exemplary embodiment of the present invention includes a process for controlling a moving web. The process involves the provision of a web that is moving at a determined speed. The web has a determined basis weight. A wave is generated in the moving web and the speed of the wave in the web is measured. The process further includes the determination of the instability index. The instability index is in a desired range, that being between about 0.6 and 1.0.
Alternatively, the immediately identified exemplary embodiment of the present invention may also be modified in accordance with the present invention where the desired range of the instability index is between about 0.6 and about 0.9, between about 0.8 and 1.0, and between 0.7 and 1.0.
Also provided in accordance with the present invention is an apparatus for measuring the instability index in a moving web. The apparatus includes an air pulse that is used to apply a pulse of fluid to the web to create a wave in the web. As least two laser displacement transducers are present for measuring the displacement of the web as the wave moves through the web. A computer is also present which obtains signals from the at least two laser displacement transducers. The computer calculates the instability index in the web based on the speed of the wave in the web and the speed of the web.
Alternatively, the present invention includes an exemplary embodiment of the apparatus as immediately discussed where the computer calculates the tension in the web while the instability index of the web is greater that about 0.8.
The present invention also includes an exemplary embodiment of a process for producing a paper web that has substantially uniform properties. The process includes the steps of providing a moving paper web, and determination of the tension on the paper web while the web is moving. The instability index of the web is greater than or equal to 0.5. Further, the process includes the step of adjusting the cross directional dryer coating of the web based on the determined tension of the web. Additionally, the creping chemistry is adjusted based on the instability index.
Another exemplary embodiment of the present invention includes a process for producing a paper web that has at least two flows of different strengths. The process includes the step of providing a moving layered paper web, and providing at least one hardwood flow and at least one softwood flow. These flows form at least one hardwood layer and at least one softwood layer of the paper web. The tension on the paper web is determined while the web is moving. The tension of the paper web is controlled by increasing the hardwood flow and decreasing the softwood flow in regions of high tension, maintaining a uniform basis weight. Further, softwood flow is increased and hardwood decreased in regions of low tension so that the tension is uniform, maintaining a uniform basis weight.
Also, the present invention includes an exemplary embodiment of the process as immediately discussed where the instability index of the web is greater than or equal to 0.5.